Tar may be described as black gold because it is dark and historically valuable, but the phrase is more commonly associated with oil. For tar, it should be used carefully as a metaphor, not as a universal historical nickname.
Black gold is a tempting phrase for tar because tar is dark, sticky, and economically useful. It helped preserve ships, ropes, fences, roofs, and other working materials. In some local economies, tar production could be a serious source of income. That makes the metaphor understandable.
Still, the phrase black gold is much more strongly associated with petroleum. If an old source, museum text, or modern article calls tar black gold, it is usually making a point about value rather than naming a precise historical category. It should not be treated as proof that everyone in the past used that phrase for tar.
Tar was valuable for practical reasons. It represented labour in the forest, heat, barrels, transport, trade relationships, and demand from shipyards or farms. Naval tar in particular could matter because fleets required reliable supplies of preservative and sealing materials.
So the answer is yes, tar can be called black gold in a loose descriptive sense. But a careful site should explain the metaphor and avoid pretending it was a fixed universal name.